However, you can make it more expressive depending on the context:
* Informal:
* "Je suis dans un mauvais jour." (I'm in a bad day.)
* "J'ai une journée pourrie." (I have a rotten day.)
* "C'est une journée de merde." (It's a shitty day.) *This is very informal and vulgar*
* Formal:
* "Je traverse une période difficile." (I'm going through a difficult period.)
You can also add details to explain why you're having a bad day:
* "J'ai une mauvaise journée, car j'ai raté mon examen." (I'm having a bad day because I failed my exam.)
Remember to adjust the tone and level of formality according to the situation.